Fanatical and humble bundle (the good old days) are good examples.

Incidentally Wolfire Games—the studio that founded Humble (but no longer operates it)—is currently in class-action litigation against Valve for this very issue.

I don’t know what you say “non-discounted”, cheaper is cheaper no matter what.

The Steam Distribution Agreement AFAIK allows temporary sales on other platforms to undercut Steam, but requires the "resting" price matches that on Steam. By specifying "non-discounted" I meant to indicate that although sales do exist on other platforms, the normal price of an item always matches on Steam. A quick few spot checks show the non-sale price of games on Humble, Steam, and Fanatical are equal.

"Cheaper is cheaper" kind of overlooks the core issue. Ultimately a publisher on Epic Games Store—which has a fee of 12% instead of Steam's 30%—can have a lower price for a game as part of a promotion, but can't just sell every game 18% cheaper always without violating Steam's terms and being risk being de-listed.

Steam doesn’t get a cut from keys sold in perfectly legal thirth party stores like fanatical, humble or gmg. Epic does not sell steam keys so obviously no.

Okay, gotcha. Yeah, I misunderstood. For Steam Keys it's pretty clear that Valve should be able to control the price since they provide the services after that key is purchased.

But the PMFN applies to all copies, even those distributed outside of Steam (e.g. the direct-from-publisher option I mentioned). Last time I was in a thread on this, another user found the following in the complaint (page 55) from the Wolfire v. Valve case mentioned above:

204. TomG also explained to another game publisher that the publisher should “[t]hink critically about how your decisions might affect Steam customers, and Valve. If the offer you’re making fundamentally disadvantages someone who bought your game on Steam, it’s probably not a great thing for us or our customers (even if you don’t find a specific rule describing precisely that scenario).” In that same thread, TomG responded to a question by stating: “we usually choose not to sell games if they’re being sold on our store at a price notably higher than other stores. That is, we’d want to get that lower base price as well, or not sell the game at all."
205. In response to one inquiry from a game publisher, in another example, Valve explained: “We basically see any selling of the game on PC, Steam key or not, as a part of the same shared PC market- so even if you weren’t using Steam keys, we’d just choose to stop selling a game if it was always running discounts of 75% off on one store but 50% off on ours. . . .”